This invention relates to a twin hull, semisubmersible floating vessel and more specifically to a semisubmersible barge mounting a heavy duty derrick or crane for use in offshore, particularly deep water, construction including, for example the erection and dismantling of oil drilling and production platforms as well as other offshore lifting and transfer functions.
Offshore activities, such as current attempts to drill and exploit oil wells at sea, have led to the development and construction of various special purpose marine structures capable of operations in the offshore environment over extended periods of time. For example, one such structure employed in offshore oil drilling operations comprises a fixed, self-contained drilling platform erected on piles driven into the sea floor, with the platform mounting a drilling rig, auxiliary equipment and crew's quarters. A variation of the foregoing structure provides a somewhat smaller platform similarly erected on piles and having a drilling rig located thereon, the auxiliary equipment and crew being located on a tender tied alongside.
To erect structures in the offshore environment as well as to dismantle the same as in the case of discontinued oil drilling and production platforms and other structures, barges mounting heavy duty derricks or cranes have been employed to lift, transfer and set into place the parts forming such structures. For example, current methods of offshore construction, particularly the construction of oil drilling and production platforms, employ such barges to drive piles at the construction site on which the platform is mounted. Present practice provides for the assembly on land of the component parts of the platform to form subassemblies which are then loaded aboard derrick or crane barges for transport to the construction site. At the site, these barges provide a work deck from which the subassemblies are offloaded by the heavy duty derricks or cranes mounted on the barges and assembled to form the completed structure.
Present derrick or crane barges employed for this purpose comprise single hull surface floating vessels which are either towed or self-propelled to and anchored at the construction site. Platform erecting and dismantling operations conducted from barges of this type are, however, highly restricted by sea state conditions, since excessive vessel motion in heave, pitch and roll precludes crane or derrick operations. For example, surface floating derrick or crane barges currently employed for offshore construction can operate in sea states having wave heights up to about 5 feet or in special cases 6 feet. The wave action against the vessel caused by sea states having wave heights in excess of these limits normally causes excessive vessel motion precluding derrick or crane operations. Construction operations utilizing present day barges are thus normally halted when these high sea state conditions are encountered and are resumed only when the sea state subsides to within the above-noted limits.
The main problems that present day vessels of this type encounter are (1) their natural period in roll, pitch and heave is inherently low and (2) their GM (distance between center of gravity and metacenter GM) is inherently high. The low natural periods are more apt to be close to the period of the waves thus causing motion amplification. The high GM values result in abrupt correcting motions when the vessels are submitted to roll or pitch excitations. This may damage the equipment, bring about structural or wire failures of the derrick due to excessive acceleration forces and cause discomfort to personnel.
Accordingly, it is a primary object of the present invention to provide a derrick barge or crane barge which minimizes the above-discussed and other shortcomings of prior vessels employed for like purposes and provides various advantages in construction, mode of operation and result over prior vessels. [The terms "derrick" or "crane" are employed hereinafter interchangeably and the vessel or barge mounting either one or the other is herein referred to as a derrick barge.]
It is another object of the present invention to provide a twin hull, semisubmersible barge mounting a heavy duty derrick for offshore construction work.
It is still another object of the present invention to provide a semisubmersible twin hull derrick barge which, particularly when in floating semisubmerged condition, has the characteristic of minimizing vessel motion due to excitation forces caused by wave action (hereinafter called "motion minimizing characteristics"). It is a related object to provide such a derrick barge affording improved motion minimizing characteristics in vessel pitch, roll and heave as well as minimizing sideslip and surge.
It is yet another object of the invention to provide a derrick barge comprising a platform and a derrick mounted in spaced relation above a pair of hulls and which can be selectively ballasted or deballasted from its normal semisubmerged floating condition to obtain even better motion minimizing characteristics when the period of the waves is the same or close to the natural period of the vessel, thereby tending to produce vessel motion amplification.
It is another related object of the present invention to provide a twin hull semisubmersible derrick barge having long natural periods in roll, pitch and heave and a lower GM in the semisubmerged condition as compared with its GM in the surface floating condition (low draft).
It is a further object of the present invention to provide a semisubmersible derrick barge having rapid mobility in transit, the ability to carry large deck loads in the surface floating condition and a beam providing for transit of the barge through the Panama Canal.
It is a still further object of the present invention to provide, in a barge mounting a heavy duty derrick, a method of coordinating operation of the derrick and the ballasting of the barge as to enable operation of the derrick when its permissible slew angle would otherwise be exceeded, and also to maintain heel angle of the vessel within limits acceptable for comfort of the crew.
These and other related objects and advantages of the present invention will become more apparent from the following specification, claims and appended drawings wherein: